1. Field of the Invention
The present invention relates to a reversible thermosensitive coloring composition comprising an electron donor compound and an electron acceptor compound, capable of inducing color formation by utilizing the coloring reaction between the electron donor compound and the electron acceptor compound. The present invention also relates to a reversible thermosensitive recording medium using the above-mentioned reversible thermosensitive coloring composition.
2. Discussion of Background
There is conventionally known a thermosensitive recording medium utilizing the coloring reaction between an electron donor compound (hereinafter referred to as a coloring agent) and an electron acceptor compound (hereinafter referred to as a color developer). This kind of thermosensitive recording medium is widely used in a variety of fields, such as in various recorders and printers for an electronic computer, a scientific measuring instrument, a facsimile apparatus, an automatic ticket vending apparatus, and a CRT medical measuring instrument. However, the coloring reaction of the conventional thermosensitive recording medium of this type has no reversibility, so that color development and decolorization cannot repeatedly be carried out.
A reversible thermosensitive recording medium capable of repeatedly recording images therein and erasing the images therefrom is proposed, which also utilizes the coloring reaction between the coloring agent and the color developer. For instance, a reversible thermosensitive recording medium comprising a mixture of gallic acid and phloroglucinol as the color developer is disclosed in Japanese Laid-Open Patent Application 60-193691. A colored material generated in this reversible thermosensitive recording medium by the application of heat thereto is removed therefrom with the application of water or water vapor thereto. However, this type of reversible thermosensitive recording medium has the shortcomings that it is difficult to impart a sufficient water resistance to the recording medium, the preservability of images recorded in the recording medium is poor, and an apparatus for removing the colored material cannot be made compact.
A rewritable optical recording medium comprising as the color developer phenolphthalein, thymolphthalein, or bisphenol is proposed as disclosed in Japanese Laid-Open Patent Application 61-237684. This type of optical recording medium produces a colored material by the steps of heating to a predetermined temperature (color development temperature) and gradually cooling, and the colored material thus generated in the recording medium is deleted therefrom when heated at a temperature higher than the color development temperature and then rapidly cooled. However, one of the drawbacks of this optical recording medium is that the coloring and decolorizing processes are complicated. In addition, the colored material is not removed from the recording medium completely through the decolorizing process, so that images with high contrast cannot be obtained.
As disclosed in Japanese Laid-Open Patent Applications 62-140881, 62-138568 and 62-138556, a reversible thermosensitive recording medium comprising a homogeneous mixture of a coloring agent, a color developer and a carboxylic acid ester is proposed. This reversible thermosensitive recording medium assumes a complete color development state at a predetermined temperature (a color development temperature), and a complete decolorization state at a predetermined temperature (a decoloriza-tion temperature) higher than the color development temperature, with each state being maintained at intermediate temperatures between the color development temperature and the decolorization temperature. Therefore, by the application of thermal energy to the recording medium using a thermal head, white images (decolorization state) can be formed on a colored background (color development state). Since the obtained images are negative as previously mentioned, the application of this type of reversible thermosensitive recording medium is restricted. In addition, it is necessary that the recorded images be maintained within a specific temperature region.
Furthermore, in reversible thermosensitive recording media disclosed in Japanese Laid-Open Patent Applications 2-188294 and 2-188293, a salt of gallic acid and higher aliphatic amine, and a salt of bis(hydroxyphenyl)acetate or bis(hydroxyphenyl)butyrate and higher aliphatic amine are respectively employed as the color developers capable of reversibly achieving the color development action and the decolorization action. The color formation is induced at a predetermined color development temperature, and the colored material generated by the color formation process is removed from each recording medium when the recording medium is heated at a temperature higher than the color development temperature. However, the color development action and the color reduction action are competitively performed in each recording medium. Accordingly, it is difficult that both of the above-mentioned actions are thermally controlled to obtain a sufficient image contrast.
As previously mentioned, the conventional reversible thermosensitive recording media utilizing the reaction between the coloring agent and the color developer have various shortcomings, and they are unsatisfactory for use in practice.
The inventors of the present invention have proposed a reversible thermosensitive coloring composition comprising a leuco compound serving as a coloring agent and an organic phosphoric acid compound with a long-chain aliphatic group, a carboxylic compound with a long-chain aliphatic group, or a phenolic compound with a long-chain aliphatic group, as a color developer, and a reversible thermosensitive recording medium comprising a recording layer which comprises the above-mentioned coloring composition, as disclosed in Japanese Patent Application 3-355078. This coloring composition can readily achieve the color development and the decolorization merely by the application of heat thereto, and the color development state and the decolorization state can be maintained at room temperature. In addition to this, the decolorization temperature is lower than the color development temperature, and the formation of images and erasure thereof can be repeatedly carried out by changing the temperature of the coloring composition.
The above-mentioned reversible thermosensitive recording medium proposed by the inventors of the present invention are incomparably advantageous over the conventional ones. However, there is the problem that the hue of a colored image formed in the recording medium is changed by the application of light thereto. In other words, the preservability of the recording medium in the color development state is poor when the recording medium is placed with being exposed to light. In addition, the decolorization action does not proceed smoothly when the recording medium in the color development state is heated at a temperature lower than the color development temperature. As previously mentioned, the problems in the quality of the recording medium remain unsolved.